The invention relates to a device for measuring a motion of a moving body.
DE 37 30 841 A1 describes a motion sensor capable of inductively measuring the velocity and acceleration of a moving body. A main magnetic field which is constant in time and penetrates a disk orthogonal to the direction of motion is produced in a locally limited partial area of the disk near the edge of the rotating, electrically conducting disk forming the moving body. For producing the main field, two opposing permanent magnets are provided along an air gap through which the disk extends. These permanent magnets are also magnetically short-circuited on the sides facing away from the disk by a yoke made out of a magnetic material, for example iron, so as to form a closed magnetic circuit. The magnetic main field of the permanent magnets induces in the moving disk locally electrical eddy currents which in turn induce a counteracting magnetic eddy current field. To measure the magnetic flux density produced by the eddy currents, a Hall effect sensor or another magnetic field sensor, for example a magneto-resistive sensor, is provided on both sides of the gap. The magnetic field sensor can determine the tangential velocity or the angular velocity of the disk. Each of the two magnetic field sensors is arranged in a gap of a corresponding flux connector ring made of magnetic material, for example iron, and simultaneously in the air gap between the two permanent magnets. The flux collector rings each define a magnetic flux path in form of a loop which extends parallel to the disk or to the rotation plane of the disk and perpendicular to the main field. Each of the flux connector rings has a straight flush segment which extends between the corresponding permanent magnets and the disk, wherein the gap with the magnetic field sensor is formed in the center of the segment, and a second segment which is connected with the first straight segment and complements the first linear segment to form a closed flux path, with the second segment projecting outwardly in the radial direction from the rotation axis into the yoke connecting the permanent magnets. Each flux collector ring in the in the center section of the U-shaped second segment that extends parallel to the first section is surrounded by a corresponding detector coil. These two detector coils measure the time variation of the flux density produced by the eddy currents and thereby provide a measurement signal for the temporal change of the tangential velocity or the rotation speed, and also for the acceleration or the acceleration in the rotation speed of the disk. Both the magnetic field sensors and the induction coils are oriented so as to measure the flux of the eddy current field which extends along the loop flux paths of the respective flux collector rings in a direction tangential to the motion direction, which is the direction perpendicular to the direction in which the main field penetrates the moving body. Accordingly, the magnetic field sensor and the induction coils are, as viewed in the motion direction, at the same height as the partial region in the moving body that is penetrated by the main magnetic field so as to measure the tangential field of the eddy currents.
One problem with the inductive motion sensor described in DE 37 30 841 A1 is heating of the moving body by Joule heating due to the induced eddy currents, with the heating effect increasing with increasing rotation speed and/or speed of the moving body. This limits the application of the conventional inductive motion sensor for measuring the rotation speed and/or speed for two reasons. On one hand, the moving body cannot exceed a maximum temperature. On the other hand, even small temperature changes can falsify the measurement result. In particular, temperature variations cause a drift of the DC magnetic field of the permanent magnets, an additional thermal noise and a drift in the output voltage of the detector coils. In addition, the signal-to-noise ratio can also change due to the aforementioned thermal drift and the coupled interference fields.
In another inductive motion sensor known from DE 29 8 13 276 U1 for measuring rotational or linear accelerations, a special soft iron support is provided for the permanent magnet(s) whose pole shoes and legs project laterally over the permanent magnets. This arrangement captures almost all the magnetic field lines produced by the eddy currents in the moving body so that sufficiently large measurement signals are obtained even with relatively small magnets and a small acceleration. Moreover, from a certain value on, the eddy currents increasingly interact with the permanent magnetic field and weaken it so that only a small residual field remains. This reduces the temperature problems associated with high rotation speed of the moving body as compared to the motion sensor known from DE 37 30 341 A1 and increases the rotation speed range.
However, the measures proposed in DE 29 8 13 276 U1 for reducing the temperature in the moving body make the sensor sensitivity dependent on the rotation speed, which can cause additional problems when evaluating the results. DE 29 8 13 276 U1 also does not provide any concrete suggestions how to detect the magnetic eddy currents fields.